Alfalfa production and winter injury due to cold

Cold temperatures and lack of snow cover are the two issues concerning alfalfa growers as they consider their 2014 crop. This winter’s extremes have had to worry alfalfa growers and caused a few questions being asked ag retailer agronomists and independent crop consultants.

Their worries may be for naught, according to Karla Hernandez, SDSU Extension forages field specialist.

"Although the alfalfa plant could die if exposed to extremely cold temperatures. In general, alfalfa plants can tolerate up to three weeks of winter injury before the plants are killed," Hernandez said.

She added that the window of safety shrinks if soil temperatures are near freezing, but is longer if the soil is colder.

"This is primarily due to the plant being forced into a deeper dormancy when the soil is colder. The plant is therefore less likely to leave dormancy in early-spring conditions, and hence becoming susceptible to an early frost," she said.

What factors affect alfalfa plants this winter?

Stand age: Older stands are more likely to winterkill than younger plants.

Soil pH: Stands in soils with a pH above 6.6 are less likely to experience winter injury.

Soil fertility: Stands planted in soils with high natural fertility are less likely to experience winter injury than those with low fertility.

Variety: Alfalfa varieties with superior winter hardiness ratings and a high disease resistance index are less likely to experience winter injury.

Cutting management: Harvest frequency and timing of fall cutting will affect alfalfa winter hardiness. The general trend shows that the shorter the interval between cuttings during the growing season, the greater risk of winter injury. An aggressive harvest schedule prevents the plant from storing carbohydrates in its root structure that it will need to maintain health as it regrows. Stands in which last cutting is taken between September 1 and middle of October in South Dakota and other northern climates are at greatest risk, as plants did not have enough time to accumulate adequate carbohydrate levels in the root system before winter.

Snow cover: Snow provides insulation to the plants and the crown. The crucial temperature region is two to four inches below the soil surface where a large part of the root structure is located. Stands that have at least six inches of stubble left will be able to retain more snow cover and be less susceptible to winter injury.

Once the snow cover melts, Hernandez encourages scouting fields and assessing them for potential problems.

What to look for when assessing winter damage:

Stands which are slow to green up: Compare your stand to other fields in the area. If you notice that some areas are starting to grow and other areas of your alfalfa field still brown, it is time to check those brown stands for injury or death.

Winter-killed roots gray appearance: If the root is soft and water can be easily squeezed from it, or it has a brown color, it is a probable sign of winter cold-related death.

Asymmetrical growth and uneven growth: These are also two indicators of winter injury. Compare the shoots on the same plant, and if you notice that one set of the shoots seems to be drastically outperforming another in terms of growth, it could be that winter cold damaged the bud structure of your plants.

Guideline on stem density:

If you think a stand has been injured, you can follow the guideline found in the table to make decisions to either keep or replant an alfalfa field after winter injury.